Is there a way to duplicate objects betwenn two objects?
If you only want to do a single object between two existing objects, you can use 3D cursor snapping. In object mode, that is respective to the object origins.

So you'd pick the two objects you'd want to place the new object in between. Then snap cursor to selected. (This places the 3D cursor at the average position of both object origins.) Then pick and duplicate an object. Then snap it to the 3D cursor location.

By the way, the default menu option for 3D cursor snapping (2.7x) is Shift+S.

Alternately you may want to look into the Oscuart Tools add-on. It allows distribution of objects. That may function more closely to what you're wanting. I'm not exactly familiar with the workflow involved there, but it'd be worth looking into.